如何只读取文件的第二行？

Question

I need to read in only every second line of a file (which is very big) so I don't want to use readlines() . I'm unsure how to implement the iterator so any suggestions are welcome. One possibility is to call next() twice. Not very appealing.

    with open(pth_file, 'rw') as pth:
        pth.next()
        for i,row in enumerate(pth):
            # do stuff with row
            pth.next()

Or creating my own iterator like

for i, row in enumerate(pth):
    if i...

Answer 1

Using itertools.islice :

import itertools

with open(pth_file) as f:
    for line in itertools.islice(f, 1, None, 2):
        # 1: from the second line ([1])
        # None: to the end
        # 2: step

        # Do something with the line

Answer 2

You can use a custom iterator:

def iterate(pth):
    for i, line in enumerate(pth, 1):
        if not i % 2:
            yield line